1. Field of the Invention
This invention relates to electro-chemical cells. More specifically, the present invention is directed to an electro-chemical cell utilizing a membrane for separating the cell components from a medium being tested.
2. Description of the Prior Art
Cells of this type are for instance used for determining the oxygen or carbon dioxide content of a fluid such as blood or its pH value. One well-known cell of this kind is the so-called Clark Cell used for measurements in biological media which cell includes a selectively permeable membrane for separating the cell components from the media. For this purpose reduction of the dimensions of such cells is desirable, in particular for measuring inside blood vessels. Since the metal of the anode as well as the electrolyte are consumed by the electro-chemical process during oxygen detection, a reasonable amount of both materials must be available in the cell. This is one of the limits for reducing the size of a Clark Cell. The required amount of electrolyte depends on the desired sensitivity of the sensor. This in turn is a function of the dimensions of the cathode and the thickness and permeability of the membrane. In most cases a small diameter of the cathode is favorable for obtaining a high-quality measuring signal in order to keep this signal unaffected by the flow conditions of the biological medium. If the resulting electrical signal, however, is too small, interference and noise signals will disturb the reliability of the measuring signal. The use of small cathodes formed as a circle, spiral or line results in a high electrical current. A high current consumes a relatively large amount of electrolyte which has to be stored in the cell. The combination of an optimum shape and extension of the cathode and the required miniaturization of the sensor, therefore, require a compromise.
The electrolyte which is necessary for the electro-chemical process in a Clark Cell is in a sufficient amount available in the biological medium. In the conventional Clark Cell one of the functions of the membrane is to define the oxygen flux to the cathode. The other function of the membrane is to separate the electrolyte in the Clark Cell from the electrolyte in the biological medium in order to avoid mixing of the two electrolytes. A direct contact between the biological medium and the cathode of the Clark Cell further would reduce the functionality of the cell because of adhesion of proteins to the surface of the cathode which would reduce the effective surface area of the cathode.
A second type of the Clark Cell is known using a porous membrane made of methycellulose, polystyrene or a hydrophylic polymer such as HEMA (hydroxyethylmetacrylate), which is simultaneously used for defining the oxygen flux on the one side and to allow the transport of the electrolyte from the biological medium into the Clark Cell. One advantage of this type of cell is the possibility to store the dry sensor over a long period. The electrolyte enters the cell as soon as it is poured into contact with the biological medium. A disadvantage of this cell, however, is that the membrane may be deformed or swell in an undefined manner which alters the oxygen flux and therewith the sensitivity of the cell. This type of a Clark Cell, therefore, has to be recalibrated relatively often.
In a third type of Clark Cell the necessary amount of electrolyte is stored as dry salt under the membrane so that this cell can be stored in dry form over a long period of time. Activation of the cell is accomplished as soon as water vapor migrates through the membrane and dissolves the salt, therewith forming the liquid electrolyte. However, for this type of Clark Cell a long activation period of about four hours is required before the cell becomes fully operable.